efi/libstub: Avoid protocol wrapper for file I/O routines

The EFI file I/O routines built on top of the file I/O firmware
services are incompatible with mixed mode, so there is no need
to obfuscate them by using protocol wrappers whose only purpose
is to hide the mixed mode handling. So let's switch to plain
indirect calls instead.

This also means we can drop the mixed_mode aliases from the various
types involved.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Borislav Petkov <bp@alien8.de>
Cc: James Morse <james.morse@arm.com>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: https://lkml.kernel.org/r/20191224151025.32482-15-ardb@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>